Quantitative analysis of the malignant potential in cancer cells is a method currently under discussion. Recently the stereological estimation of cancer cells has been utilized in making an objective and quantitative pathological diagnosis. In this study, we estimated the mean nuclear volume (MNV) of untreated bladder carcinomas in 128 patients by stereological methods and attempted to quantitatively analyze the malignant potential of the carcinoma. The MNV was significantly enlarged as the tumor advanced in grade and stage. MNV was largest in grade 3 tumors (340.2 +/- 100.1 microns3) followed by grade 2 tumours (206.2 +/- 90.6 microns3) (P less than 0.01), and grade 1 tumors (130.6 +/- 46.7 microns3) (P less than 0.01). MNV was larger in pT1 tumors (278.2 +/- 126.9 microns3) than in pTa tumors (156.9 +/- 60.5 microns3) (P less than 0.01). MNV was also larger in invasive tumors (T2, T3 and T4: 318.2 +/- 104.0 microns3) than in superficial tumors (Ta and T1: 203.5 +/- 109.2 microns3) (P less than 0.01). Patients were then divided into two subgroups, one with large nuclei (MNV greater than or equal to 197.3 microns3), and the other with small nuclei (MNV less than 197.3 microns3). Survival and disease-free rates in patients with small nuclei (5-year survival rate: 92.9%, 5-year disease-free rate: 24.4%) were significantly better than in patients with large nuclei (5-year survival rate: 58.0%, 5-year disease-free rate: 12.5%). For patients with grade 2 tumors, those with small nuclei had a good survival rate (5-year survival rate: 95.5%), similar to that of patients with grade 1 tumors (5-year survival rate: 95.0%).(ABSTRACT TRUNCATED AT 250 WORDS)